It is known that monoalkenyl aromatic polymers such as polystyrene have improved toughness when a small amount of a rubber polymer is incorporated therein. These rubber-modified compounds are resistant to breakage under impact and are consequently useful in making molded articles. High impact polystyrene is obtained, for example, by polymerizing a solution in styrene of a minor amount, one to fifteen parts by weight per hundred parts of styrene, of an unsaturated rubbery polymer of a conjugated diolefin, e.g., polybutadiene or a copolymer of styrene and butadiene. While high impact polystyrene possesses good mechanical properties such as impact strength, hardness, heat distortion temperature, other mechanical properties such as elongation and temperature-viscosity relationships are less than desirable for molding operations. These latter properties affect the rheological behavior of the product and, accordingly, its processability. It is known that certain additives, known as flow promoters, can be introduced into high impact polystyrene in order to give the desired properties of flow, prevent sticking to the mold, and the like. These additives, however, have a detrimental effect on other mechanical properties.
U.S. Pat. No. 3,506,740 discloses the use of amorphous polymers selected from amorphous, atactic low molecular weight polypropylene and polybutylene as flow promoters. Likewise, U.S. Pat. No. 4,042,551 discloses the use of certain polyethylene waxes as flow promoters. While these polymers have resulted in improved flow properties, their use has also resulted in a drop in heat resistence as measured by the Vicat temperature reading.
It has now been discovered that a certain oxidized polyethylene material when added as a flow promoter to a high impact polystyrene molding composition increases melt flow but does not reduce heat resistence.